Method and apparatus for creating a virtual electrode used for the ablation of tissue

ABSTRACT

A method and apparatus for creating a virtual electrode to ablate bodily tissue. The surgical apparatus includes an inner tube and an outer tube. The inner tube defines a proximal portion and a distal portion. The distal portion forms an orifice for distributing a conductive solution from the inner tube and further forms an electrode. The outer tube coaxially receives the inner tube such that the outer tube is slidable relative to the inner tube. With this configuration, the outer tube selectively blocks flow of conductive solution from the orifice. During use, conductive solution distributed from the orifice is subjected to a current from the electrode, thereby creating a virtual electrode.

This application claims the benefit of U.S. Provisional Application No.60/091,948, filed on Jul. 7, 1998, now abandoned.

FIELD OF THE INVENTION

The present invention relates generally to an apparatus for creating avirtual electrode. More particularly, the present invention relates toan apparatus for the creation of a virtual electrode that is useful forthe ablation of soft tissue and neoplasms.

BACKGROUND OF THE PRESENT INVENTION

The utilization of an electric current to produce an ameliorative effecton a bodily tissue has a long history, reportedly extending back to theancient Greeks. The effects on bodily tissue from an applied electriccurrent, and thus the dividing line between harmful and curativeeffects, will vary depending upon the voltage levels, current levels,the length of time the current is applied, and the tissue involved. Onesuch effect resulting from the passage of an electric current throughtissue is heat generation.

Body tissue, like all non-superconducting materials, conducts currentwith some degree of resistance. This resistance creates localizedheating of the tissue through which the current is being conducted. Theamount of heat generated will vary with the power P deposited in thetissue, which is a function of the product of the square of the currentI and the resistance R of the tissue to the passage of the currentthrough it (P=I²R.).

As current is applied to tissue, then, heat is generated due to theinherent resistance of the tissue. Deleterious effects in the cellsmaking up the tissue begin to occur at about 42° Celsius. As thetemperature of the tissue increases because of the heat generated by thetissue's resistance, the tissue will undergo profound changes andeventually, as the temperature becomes high enough, that is, generallygreater than 45° C., the cells will die. The zone of cell death is knownas a lesion and the procedure followed to create the lesion is commonlycalled an ablation. As the temperature increases beyond cell deathtemperature, complete disintegration of the cell walls and cells causedby boiling off of the tissue's water can occur. Cell death temperaturescan vary somewhat with the type of tissue to which the power is beingapplied, but generally will begin to occur within the range of 45° to60° C., though actual cell death of certain tissue cells may occur at ahigher temperature.

In recent times, electric current has found advantageous use in surgery,with the development of a variety of surgical instruments for cuttingtissue or for coagulating blood. Still more recently, the use ofalternating electric current to ablate, that is, kill, various tissueshas been explored. Typically, current having a frequency from about 3kilohertz to about 300 gigahertz, which is generally known asradiofrequency or radiofrequency (RF) current, is used for thisprocedure. Destruction, that is, killing, of tissue using an RF currentis commonly known as radiofrequency ablation. Often radiofrequencyablation is performed as a minimally invasive procedure and is thusknown as radiofrequency catheter ablation because the procedure isperformed through and with the use of a catheter. By way of example,radiofrequency catheter ablation has been used to ablate cardiac tissueresponsible for irregular heartbeats or arrhythmias.

The prior art applications of current to tissue have typically involvedapplying the current using a “dry” electrode. That is, a metal electrodeis applied to the tissue desired to be affected and a generated electriccurrent is passed through the electrode to the tissue. A commonly knownexample of an instrument having such an operating characteristic is anelectrosurgical instrument known as a “bovie” knife. This instrumentincludes a cutting/coagulating blade electrically attached to a currentgenerator. The blade is applied to the tissue of a patient and thecurrent passes through the blade into the tissue and through thepatient's body to a metal base electrode or ground plate usually placedunderneath and in electrical contact with the patient. The baseelectrode is in turn electrically connected to the current generator soas to provide a complete circuit.

As the current from the bovie knife passes from the blade into thetissue, the resistance provided by the tissue creates heat. In thecutting mode, a sufficient application of power through the bovie knifeto the tissue causes the fluid within the cell to turn to steam,creating a sufficient overpressure so as to burst the cell walls. Thecells then dry up, desiccate, and carbonize, resulting in localizedshrinking and an opening in the tissue. Alternatively, the bovie knifecan be applied to bleeding vessels to heat and coagulate the bloodflowing therefrom and thus stop the bleeding.

As previously noted, another use for electrical instruments in thetreatment of the body is in the ablation of tissue. To expand further onthe brief description given earlier of the ablation of cardiac tissue,it has long been known that a certain kind of heart tissue known assino-athial and atrio-ventricular nodes spontaneously generate anelectrical signal that is propagated throughout the heart alongconductive pathways to cause it to beat. Occasionally, certain hearttissue will “misfire”, causing the heart to beat irregularly. If theerrant electrical pathways can be determined, the tissue pathways can beablated and the irregular heartbeat remedied. In such a procedure, anelectrode is placed via a catheter into contact with the tissue and thencurrent is applied to the tissue via the electrode from a generator ofRF current. The applied current will cause the tissue in contact withthe electrode to heat. Power will continue to be applied until thetissue reaches a temperature where the heart tissue dies, therebydestroying the errant electrical pathway and the cause of the irregularheartbeat.

Another procedure using RF ablation is transurethral needle ablation, orTUNA, which is used to create a lesion in the prostate gland for thetreatment of benign prostatic hypertrophy (BPH) or the enlargement ofthe prostate gland. In a TUNA procedure, a needle having an exposedconductive tip is inserted into the prostate gland and current isapplied to the prostate gland via the needle. As noted previously, thetissue of the prostate gland heats locally surrounding the needle tip asthe current passes from the needle to the base electrode. A lesion iscreated as the tissue heats and the destroyed cells may be reabsorbed bythe body, infiltrated with scar tissue, or just become non-functional.

While there are advantages and uses for such “dry” electrodeinstruments, there are also several notable disadvantages. One of thesedisadvantages is that during a procedure, coagulum—dried blood cells andtissue cells—will form on the electrode engaging the tissue. Coagulumacts as an insulator and effectively functions to prevent currenttransfer from the blade to the tissue. This coagulum “insulation” can beovercome with more voltage so as to keep the current flowing, but onlyat the risk of arcing and injuring the patient. Thus, during surgerywhen the tissue is cut with an electrosurgical scalpel, a build-up ofcoagulated blood and desiccated tissue will occur on the blade,requiring the blade to be cleaned before further use. Typically,cleaning an electrode/scalpel used in this manner will involve simplyscraping the dried tissue from the electrode/scalpel by rubbing thescalpel across an abrasive pad to remove the coagulum. This is a tediousprocedure for the surgeon and the operating staff since it requires the“real” work of the surgery to be discontinued while the cleaningoperation occurs. This procedure can be avoided with the use ofspecially coated blades that resist the build up of coagulum. Suchspecialty blades are costly, however.

A second disadvantage of the dry electrode approach is that theelectrical heating of the tissue creates smoke that is now known toinclude cancer-causing agents. Thus, preferred uses of such equipmentwill include appropriate ventilation systems, which can themselvesbecome quite elaborate and quite expensive.

A further, and perhaps the most significant, disadvantage of dryelectrode electrosurgical tools is revealed during cardiac ablationprocedures. During such a procedure, an electrode that is otherwiseinsulated but having an exposed, current carrying tip is inserted intothe heart chamber and brought into contact with the inner or endocardialside of the heart wall where the ablation is to occur. The current isinitiated and passes from the current generator to the needle tipelectrode and from there into the tissue so that a lesion is created.Typically, however, the lesion created by a single insertion isinsufficient to cure the irregular heartbeat because the lesion createdis of an insufficient size to destroy the errant electrical pathway.Thus, multiple needle insertions and multiple current applications arealmost always required to ablate the errant cardiac pathway, prolongingthe surgery and thus increasing the potential risk to the patient.

This foregoing problem is also present in TUNA procedures, whichsimilarly require multiple insertions of the needle electrode into theprostate gland. Failing to do so will result in the failure to create alesion of sufficient size otherwise required for beneficial results. Aswith radiofrequency catheter ablation of cardiac tissue, then, theability to create a lesion of the necessary size to alleviate BPHsymptoms is limited and thus requires multiple insertions of theelectrode into the prostate.

A typical lesion created with a dry electrode using RF current and asingle insertion will normally not exceed one centimeter in diameter.This small size—often too small to be of much or any therapeuticbenefit—stems from the fact that the tissue surrounding the needleelectrode tends to desiccate as the temperature of the tissue increases,leading to the creation of a high resistance to the further passage ofcurrent from the needle electrode into the tissue, all as previouslynoted with regard to the formation of coagulum on an electrosurgicalscalpel. This high resistance—more properly termed impedance sincetypically an alternating current is being used—between the needleelectrode and the base electrode is commonly measured by the RF currentgenerator. When the measured impedance reaches a pre-determined level,the generator will discontinue current generation. Discontinuance of theablation procedure under these circumstances is necessary to avoidinjury to the patient.

Thus, a typical procedure with a dry electrode may involve placing theneedle electrode at a first desired location; energizing the electrodeto ablate the tissue; continue applying current until the generatormeasures a high impedance and shuts down; moving the needle to a newlocation closely adjacent to the first location; and applying currentagain to the tissue through the needle electrode. This cycle ofelectrode placement, electrode energization, generator shut down,electrode re-emplacement, and electrode re-energization, will becontinued until a lesion of the desired size has been created. As noted,this increases the length of the procedure for the patient.Additionally, multiple insertions increases the risk of at least one ofthe placements being in the wrong location and, consequently, the riskthat healthy tissue may be undesirably affected while diseased tissuemay be left untreated. The traditional RF ablation procedure of using adry ablation therefore includes several patient risk factors that bothpatient and physician would prefer to reduce or eliminate.

The therapeutic advantages of RF current could be increased if a largerlesion could be created safely with a single positioning of thecurrent-supplying electrode. A single positioning would allow theprocedure to be carried out more expeditiously and more efficiently,reducing the time involved in the procedure. Larger lesions can becreated in at least two ways. First, simply continuing to apply currentto the patient with sufficiently increasing voltage to overcome theimpedance rises will create a larger lesion, though almost always withundesirable results to the patient. Second, a larger lesion can becreated if the current density, that is, the applied electrical energy,could be spread more efficiently throughout a larger volume of tissue.Spreading the current density over a larger tissue volume wouldcorrespondingly cause a larger volume of tissue to heat in the firstinstance. That is, by spreading the applied power throughout a largertissue volume, the tissue would heat more uniformly over a largervolume, which would help to reduce the likelihood of generator shutdowndue to high impedance conditions. The applied power, then, will causethe larger volume of tissue to be ablated safely, efficiently, andquickly.

Research conducted under the auspices of the assignee of the presentinvention has focused on spreading the current density throughout alarger tissue volume through the creation, maintenance, and control ofa—virtual electrode—within or adjacent to the tissue to be ablated. Avirtual electrode can be created by the introduction of a conductivefluid, such as isotonic or hypertonic saline, into or onto the tissue tobe ablated. The conductive fluid will facilitate the spread of thecurrent density substantially equally throughout the extent of the flowof the conductive fluid, thus creating an electrode—a virtualelectrode—substantially equal in extent to the size of the deliveredconductive fluid. RF current can then be passed through the virtualelectrode into the tissue.

A virtual electrode can be substantially larger in volume than theneedle tip electrode typically used in RF interstitial ablationprocedures and thus can create a larger lesion than can a dry, needletip electrode. That is, the virtual electrode spreads or conducts the RFcurrent density outward from the RF current source—such as a currentcarrying needle, forceps or other current delivery device—into or onto alarger volume of tissue than is possible with instruments that rely onthe use of a dry electrode. Stated otherwise, the creation of thevirtual electrode enables the current to flow with reduced resistance orimpedance throughout a larger volume of tissue, thus spreading theresistive heating created by the current flow through a larger volume oftissue and thereby creating a larger lesion than could otherwise becreated with a dry electrode.

While the efficacy of RF current ablation techniques using a virtualelectrode has been demonstrated in several studies, the currentlyavailable instruments useful in such procedures lags behind the researchinto and development of hoped-for useful treatment modalities for theablation of soft tissue and malignancies.

It would be desirable to have an apparatus capable of creating a virtualelectrode for the controlled application of tissue ablating RF electriccurrent to a tissue of interest so as to produce a lesion of desiredsize and configuration.

SUMMARY OF THE INVENTION

One aspect of the present invention provides a surgical apparatus forcreating a virtual electrode for ablating bodily tissue. The surgicalapparatus includes an inner tube and an outer tube. The inner tubedefines a proximal portion and a distal portion. The distal portionforms an orifice for distributing a conductive solution from the innertube and further forms an electrode. The outer tube coaxially receivesthe inner tube. More particularly, the outer tube is slidable relativeto the inner tube such that the outer tube selectively blocks theorifice.

Another aspect of the present invention relates to a surgical system forcreating a virtual electrode for ablating bodily tissue. The surgicalsystem includes a fluid source, a current source and a surgicalinstrument. The fluid source maintains a supply of a conductivesolution. The current source is configured to generate an electricalcurrent. The surgical instrument includes an inner tube, an outer tubeand an electrode. The inner tube is fluidly connected to the fluidsource and defines a proximal portion and a distal portion, with thedistal portion forming an orifice for releasing the conductive solutionfrom the inner tube. The outer tube is coaxially disposed over the innertube such that the outer tube is slidable relative to the inner tube toselectively expose the orifice. Finally, the electrode is associatedwith the distal portion and is electrically connected to the currentsource.

Another aspect of the present invention relates to a method of forming avirtual electrode for ablating bodily tissue. The method includesproviding a surgical instrument including an inner tube slidablyreceived within an outer tube. The inner tube defines a proximal portionand a distal portion, the distal portion forming an orifice fordistributing a conductive solution from the inner tube and furtherforming an electrode. The distal portion is delivered to a target site.The outer tube is positioned relative to the inner tube such that theorifice is exposed. Conductive solution is distributed from the innertube via the orifice. The outer tube is repositioned relative to theinner tube such that the orifice is blocked. Finally, a current isapplied to the distributed conductive solution via the electrode tocreate a virtual electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a virtual electrode ablation system inaccordance with the present invention;

FIG. 2 is a perspective view of a surgical apparatus, with portions cutaway, in accordance with the present invention;

FIG. 3 is an enlarged, side view of a distal portion of the surgicalapparatus of FIG. 2;

FIGS. 4A and 4B are schematic views of a portion of the surgicalapparatus of FIG. 2;

FIG. 4C is an exploded, perspective view of a portion of the surgicalapparatus of FIG. 2; and

FIGS. 5-8 are side views of a surgical apparatus in accordance with thepresent invention, depicting various positions of an outer tube relativeto an inner tube.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates in block form a system 10 for RF ablation useful withthe present invention. The system 10 includes a source of radiofrequencyalternating electric current 12, a source of RF ablating fluid 14,including but not limited to saline and other conductive solutions, anda surgical instrument 16 for delivering RF current and ablation fluid toa tissue site (not shown) for ablation purposes. In one preferredembodiment, the surgical instrument 16 is connected to the currentsource 12 and the fluid source 14. It will be understood that thecurrent source 12 and the fluid source 14 may be combined into a singleoperational structure controlled by an appropriate microprocessor for acontrolled delivery of ablating fluid and a controlled application of RFcurrent, both based upon measured parameters such as but not limited to,flow rate, tissue temperature at the ablation site and at areassurrounding the ablation site, impedance, the rate of change of theimpedance, the detection of arcing between the surgical instrument andthe tissue, the time period during which the ablation procedure has beenOperating, and additional factors as desired.

While the surgical instrument 16 is shown as being connected to both thecurrent source 12 and the fluid source 14, the present system is not solimited but could include separate needles or other instruments usefulin RF liquid ablation procedures, that is, for example, a singlestraight or coiled needle having an exposed end and a fluid flow paththere through could be used to deliver both fluid and current to thetarget tissue for ablation purposes. Alternatively, a separate needlecould be used to deliver the current and a separate needle or needlescould be used to deliver fluid to the target tissue. In addition, theapplication of the present system is not limited to the use of straightneedles or helical needles as surgical instruments but could find usewith any type of instrument wherein a conductive solution is deliveredto a tissue and an RF current is applied to the tissue through theconductive fluid. Such instruments thus would include straight needles,helical needles, forceps, roller balls, instruments for the treatment ofvascular disorders, and any other instrument.

In one preferred embodiment, the system 10 further includes a secondfluid source 18 for delivery of tissue protecting fluid via a deliveryinstrument 20, to a tissue whose ablation is not desired.

The surgical instrument 16 may assume a wide variety of forms. Onepreferred embodiment of a surgical apparatus 30 useful in an RF ablationprocedure is shown in FIG. 2. The apparatus 30 includes an outer thinwalled tube 32, an inner thinner walled tube 34, and may, if desired,include an inner stylet or probe 36. Tubes 32 and 34 and stylet 36 aresubstantially coaxially mounted relative to each other and are movablein the proximal-distal direction relative to each other.

The outer tube 32 preferably includes a collar 38 with a control knob 40attached at a proximal end thereof. The outer tube 32 further preferablyincludes at least one slot or aperture 42 located at a distal endthereof. If desired, multiple apertures 42 may be disposed in anydesired manner about the circumference of the distal end of the outertube 32. The aperture 42 may take on multiple configurations. In theembodiment shown in FIG. 2, the aperture 42 has an elongated oval orelliptical configuration.

In one preferred embodiment, the inner tube 34 is configured as a needleelectrode. The inner tube 34, which is slidably received within theouter tube 32, preferably includes a shutter index element 44 attachedadjacent a proximal end thereof. The shutter index element 44 will bediscussed in further detail below. Attached to a proximal end of theinner tube 34 is a hemostasis valve 46 having a port 48 through which RFablating fluid, such as but not limited to saline and other conductivesolutions, may be supplied from the fluid source 14 (FIG. 1) asindicated by arrow 50. A distal end of the inner tube 34 includes aplurality of orifices or apertures 52 of varying sizes and shapes asdesired. As shown in FIG. 2, there are preferably four sets of theapertures 52 equally spaced about a circumference of the distal end ofthe inner tube 34. In one preferred embodiment, each set includes fiveapertures increasing in size toward a center of the individual set.Thus, during use, as fluid flows through the apertures 52, more of thefluid will flow through the center aperture than will flow through theother apertures. As a result, the bolus of the fluid forming at aparticular target site may take on a roughly elliptical shape, makingallowances for the tissue at the target site and the vasculature thatmay be implicated by any particular placement.

The inner tube 34 includes at its most distal end an exposed electrode54 through which RF current can be applied to the tissue. Thus, theinner tube 34 may be metallic or otherwise conductive and insulatedalong its length except for the exposed electrode 54. The electrode 54is preferably electrically connected to the current source 12 (FIG. 1).

FIG. 3 illustrates a plan view of the distal end of the inner tube orneedle electrode 34. It will be understood that the needle electrode 34will be insulated except at the distal end to prevent electrical currentfrom flowing into the tissue (not shown) at any location except from thedistal end thereof.

Returning to FIG. 2, the probe 36 may take the form of an inner stylet,preferably having a thermocouple (not shown) disposed at the distal endthereof. Because the probe 36 is movable relative to the electrode 54,the thermocouple may be placed at a desired location away from theelectrode to monitor tissue temperature.

FIGS. 4A-4C illustrate in greater detail the connections between thecontrol knob 40 and the outer tube 32 on the one hand, and the shutterindex element 44 and the inner tube 34 on the other. The control knob 40is preferably connected to the collar 38 of the outer tube 32 by asupport 56. The outer tube 32 and the control knob 40 are rotatable andaxially movable relative to the inner tube 34 and the shutter indexelement 44. Thus, the control knob 40 can be selectively moved into andout of engagement with a portion of the shutter index element 44. Forexample, the control knob 40 may be rotated from a first engagementposition (FIG. 4A), and then moved axially to a second engagementposition (FIG. 4B). The outer tube 32 is maneuvered in conjunction withthe control knob 40, and thus moves relative to the inner tube 34 withmovement of the control knob 40. For example, as best shown in FIG. 4C,the shutter index element 44 is preferably configured to form axialslots 58 a-58 d sized to receive the support 56. Each of the axial slots58 a-58 d is connected to at least one circumferential slot, similarlysized to receive and selectively maintain the support 56. For example,the axial slot 58 a is shown as being connected to three circumferentialslots 60, whereas the axial slot 58 b is connected to onecircumferential slot 62. Any other number of circumferential slots isequally acceptable and dictates a desired position(s) of the outer tube32 relative to the inner tube 34, as described in greater detail below.

Translation of the control knob 40/outer tube 32 relative to the shutterindex element 44/inner tube 34 and the effect on fluid flow is shown ingreater detail in FIGS. 5-8. In each of FIGS. 5-8, the control knob 40(FIG. 4A) has not been shown for purposes of clarity. Instead, only thesupport 56, which extends from the control knob 40 to the collar 38(FIG. 4A), has been depicted. Further, the shutter index element 44 isshown as including the axial slot 58 a connected to four circumferentialslots 64 a-64 d. As previously described, the support 56 can bemaneuvered along the axial slot 58 a into selective engagement with eachof the circumferential slots 64 a-64 d, thereby locating the support 56within any one of the circumferential slots 64 a-64 d and resulting in adefined relationship of the outer tube 32 relative to the inner tube 34.To assist a user in positioning the support 56, the shutter indexelement 44 may further include indicia 66 a-66 d associated with thecircumferential slots 64 a-64 d, respectively. The indicia 66 a-66 d mayassume a wide variety of forms for providing a user with an indicationof outer tube 32/inner tube 34 positioning. For example, the indicia 66a may be a filled circle (“”) representing that all of the orifices 52of the inner tube 34 are open; the indicia 66 b may be a half filledcircle (“”) representing the orifices 52 being partially open; theindicia 66 c may be an open circle (“∘”) representing all of theorifices 52 being closed; and the indicia 66 d may be a dash (“—”)representing the slot 42 of the outer tube 32 being open or aligned withthe orifices 52.

FIG. 5 illustrates the situation where the control knob 40 (FIG. 4A) hasbeen moved to a position where the orifices 52 are fully opened andallow RF fluid to flow freely therefrom. That is to say, the support 56is positioned within the circumferential slot 64 a. At this location, adistal end of the outer tube 32 is proximal the orifices 52 and theelectrode 54. Fluid from the fluid source 14 (FIG. 1) is thereby allowedto flow from the orifices 52.

FIG. 6 illustrates the partial blocking of the orifices 52. In thisinstance, relative to FIG. 5, the control knob 40 (FIG. 4A) has beenrotated relative to the shutter index element 42 and moved distallyalong the axial slot 58 a so as to move the outer tube 32 distallyrelative to the inner tube 34. The control knob 40 has then been rotatedradially to lodge the support 56 in the circumferential slot 64 b.Movement of the outer tube 32 in this manner (e.g., distally) causes theouter tube 32 to partially block at least some of the orifices 52 andthus restrict or stop RF fluid flow therefrom, as identified by theindicia 66 b.

FIG. 7 illustrates the complete blocking of RF fluid flow from all ofthe apertures 52. More particularly, the support 56 has been moved intothe circumferential slot 66 c, resulting in a distal end of the outertube 32 being distal the orifices 52. In FIG. 8, the slot 42 at thedistal end of outer tube 32 has been moved such that fluid flows onlyfrom the orifices 52 and then through the slot 42, thus providing inessence a single aperture along a longitudinal length of the outer tube32 rather than discrete multiple apertures. The indicia 66 d providesvisual notice of this relationship to the user.

The present invention allows an operator to block the distribution ofthe RF fluid from the apparatus 30 in a selective manner. This allowsthe operator to control the volume of the fluid flow and the shape ofthe bolus or virtual electrode produced in the tissue with some degreeof latitude as allowed by the tissue structure in which the apparatus isplaced.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. A surgical apparatus for creating a virtualelectrode to ablate bodily tissue, the surgical apparatus comprising: aninner tube defining a proximal portion and a distal portion, the distalportion forming a plurality of orifices including a proximal-mostorifice and a distal-most orifice for distributing a conductive solutionfrom the inner tube and further forming an electrode; and an outer tubecoaxially disposed about the inner tube, the outer tube defining aproximal end and a distal end and forming a slot proximal the distalend, wherein the outer tube is slidable relative to the inner tube and alongitudinal spacing between a distal side of the slot and the distalend is not less than a longitudinal distance between the proximal-mostand distal-most orifices such that a portion of the outer tube distalthe slot selectively, simultaneously blocks the plurality of orifices.2. The surgical apparatus of claim 1, wherein the electrode is formeddistal the distal-most orifice.
 3. The surgical apparatus of claim 1,the slot being configured such that when aligned with the plurality oforifices, the slot dictates a desired bolus shape of the conductivesolution distributed from the inner tube.
 4. The surgical apparatus ofclaim 1, wherein the proximal portion of the inner tube is attached to ashutter index element for dictating a desired position of the outer tuberelative to the inner tube.
 5. The surgical apparatus of claim 4,wherein the outer tube defines a proximal end and a distal end, thesurgical apparatus further comprising: a support connected to theproximal end of the outer tube, the support being sized to selectivelyengage a portion of the shutter index element.
 6. The surgical apparatusof claim 5, wherein the shutter index element forms an axial slot sizedto guide the support and a circumferential slot sized to selectivelymaintain the support, the circumferential slot being located to dictatea desired orientation of the outer tube relative to the inner tube. 7.The surgical apparatus of claim 6, wherein the shutter index elementforms a plurality of spaced circumferential slots connected to the axialslot, each of the circumferential slots configured to selectivelyreceive the support to dictate a desired position of the outer tuberelative to the inner tube.
 8. The surgical apparatus of claim 1,wherein a longitudinal length of the slot is not less than alongitudinal distance between the proximal-most and distal-mostorifices.
 9. The surgical apparatus of claim 1, wherein a longitudinalspacing between the distal side of the slot and the distal end of theouter tube is not greater than a longitudinal distance between thedistal-most orifice and a distal end of the inner tube.
 10. A surgicalapparatus for creating a virtual electrode to ablate bodily tissue, thesurgical apparatus comprising: an inner tube defining a proximal portionand a distal portion, the distal portion forming a plurality of orificesfor distributing a conductive solution from the inner tube and furtherforming an electrode, wherein the plurality of orifices includes a setof axially arranged orifices increasing in size toward a center of theset; an outer tube coaxially disposed about the inner tube, wherein theouter tube is slidable relative to the inner tube such that the outertube selectively blocks at least one of the plurality of orifices.
 11. Asurgical system for creating a virtual electrode to ablate bodilytissue, the surgical system comprising: a fluid source maintaining asupply of conductive solution; a current source configured toselectively supply an electrical current; and a surgical instrumentincluding: an inner tube fluidly connected to the fluid source, theinner tube defining a proximal portion and a distal portion, wherein thedistal portion forms a plurality of orifices for releasing theconductive solution from the inner tube, an outer tube coaxiallydisposed about the inner tube, wherein the outer tube is slidablerelative to the inner tube from a first position in which the outer tubeexposes at least one of the orifices to a second position in which theouter tube blocks release of the conductive fluid from the surgicalinstrument by directly covering all of the orifices formed by the innertube, an electrode associated with the distal portion of the inner tube,the electrode being electrically connected to the current source toenergize conductive solution released from the inner tube.
 12. Thesurgical system of claim 1, wherein the electrode is formed at thedistal portion of the inner tube.
 13. The surgical system of claim 11,wherein the outer tube defines a proximal end and a distal end, andfurther wherein the outer tube forms a slot proximal the distal end, theslot being configured such that when aligned with the orifice, the slotdictates a desired bolus shape of conductive solution released from theinner tube.
 14. The surgical system of claim 11, wherein the proximalportion of the inner tube is attached to a shutter index element fordictating a desired position of the outer tube relative to the innertube.
 15. The surgical system of claim 14, wherein the outer tubedefines a proximal end and a distal end, the surgical instrument furtherincluding: a support connected to the proximal end of the outer tube,the support being sized to selectively engage a portion of the shutterindex element.
 16. The surgical system of claim 15, wherein the shutterindex element forms an axial slot sized to guide the support and acircumferential slot sized to selectively maintain the support, thecircumferential slot being located to dictate a desired orientation ofthe outer tube relative to the inner tube.
 17. The surgical system ofclaim 16, wherein the shutter index element forms a plurality of spacedcircumferential slots connected to the axial slot, each of thecircumferential slots configured to selectively receive the support soas to dictate a desired position of the outer tube relative to the innertube.
 18. The surgical system of claim 11, wherein the surgicalinstrument is configured such that in the second position, the distalend of the outer tube is proximal a distal end of the inner tube.
 19. Amethod of forming a virtual electrode for ablating bodily tissue, themethod comprising: providing a surgical instrument including an innertube slidably received within an outer tube, the inner tube defining aproximal portion and a distal portion, the distal portion forming anorifice for distributing a conductive solution from the inner tube andfurther forming an electrode; delivering the distal portion of the innertube to a target site; positioning the outer tube relative to the innertube such that the orifice is exposed; distributing a conductivesolution at the target site via the orifice; repositioning the outertube relative to the inner tube after distributing the conductivesolution such that the orifice is blocked; and applying a current to theconductive solution distributed from the orifice via the electrode tocreate a virtual electrode.
 20. The method of claim 19, wherein thedistal portion of the inner tube forms a plurality of orifices, andwherein repositioning the outer tube relative to the inner tube includesorientating the outer tube to block a portion of the plurality oforifices.
 21. The method of claim 19, wherein the outer tube defines aproximal end and a distal end and includes a slot adjacent the distalend, and wherein positioning the outer tube relative to the inner tubeincludes aligning the slot with the orifice to produce a desired bolusshape of the conductive solution.
 22. The method of claim 19, whereinthe distal portion of the inner tube forms a plurality of orifices, andwherein repositioning the outer tube relative to the inner tube includesorienting the outer tube to block all of the orifices formed by theinner tube.